Humor

Humor and stories are great but they should arise naturally out of the material, not something 'pasted on'. I worked with a talented speaker this week and my only concern was that he seemed too serious. So on his second attempt he started with a quick joke, paused, and then went right back to his serious presentation. It didn't work because his joke wasn't natural.

You speech has one theme and everything you do within that speech has to support it. Anything else gets in the way of you making your point. Don't let stories or humor sidetrack your audience. Now that being said, using levity to brighten the mood is almost always welcome.